This invention relates to producing mica tape insulation for high voltage coils of motors, generators or other electric machines. The mica tape is generally bound together by a catalyzed epoxy resinous adhesive. The catalyst is needed to promote the polymerization of the epoxy resin to a thermoset state.
The resinous adhesive in the mica tape must not gel during months of storage at room temperature. If the adhesive were to gel, air pockets would be sealed by insulating varnish which may be vacuum impregnated into the tape in a subsequent step, resulting in lower electric strength and lower corona starting levels. The mica tape adhesive must, in addition, withstand coil drying for several hours at 100.degree. C without curing.
When the adhesive is used to fully load the tape, so that subsequent impregnation with an insulating varnish is not necessary, even a slight amount of cure will make the tape too stiff to handle or wrap around a coil. The adhesive must polymerize to a thermoset state only upon final curing of the mica tape wound coil. In addition to good shelf life, the adhesive must provide good electrical properties, thermal stability, moisture resistance, pliability, and adherability without tackiness.
Heretofore, numerous catalysts have been used for mica tape epoxy resin adhesives, including, polyamines, anhydrides, polybasic acids, borate-titanates and amine-polyborate esters, as taught by Rogers in U.S. Pat. No. 3,254,150, and salts of octonic acid such as zinc octolate, as taught by Mertens in U.S. Pat. No. 3,556,925. Only boron trifluoride-amine complexes have been combined with epoxy resins to provide a mica tape adhesive that will not start to gel during storage, and that will cure only during the final high temperature bake. However, the boron trifluoride-amine catalyst may increase the power factor of the mica insulation to over 40% at the operating temperature of the electrical machine, generally about 150.degree. C. This high power factor limits the use of these adhesives in high voltage insulation i.e. over 7,500 volts, and presents some commercial problems when it is used in even lower voltage apparatus.
There is a need then for an improved epoxy-catalyst adhesive system, for use in mica tape insulation for conductors and for coils in electrical machines. This adhesive should have superior electrical properties so that it can be used for the dual purpose of sole insulating resin, as well as adhesive binder, in a preimpregnated mica tape; or solely as a binder in a mica tape that is to be subsequently impregnated with, for example, a polyester, epoxy or styrene-epoxy solventless varnish.